1. Field of the Invention
The present invention relates to an optical modulation system for modulating RF signals onto an optical carrier and more particularly to a Mach-Zehnder optical modulator (MZM) which utilizes two lasers with two optical wavelengths with two corresponding bias points to provide relatively improved performance relative to known optical modulators.
2. Description of the Prior Art
Analog optical links are generally known in the art. Such analog optical links are used in optical communications systems, particularly those systems which require bandwidth efficiency. An example of such a system is cable television (CATV) system. In such systems, a number of video channels are known to be transmitted through optical fibers. As such, RF video signals are modulated onto an optical carrier by way of an optical modulator. Various analog optical systems are known in the art. Other examples of known analog optical systems are illustrated in FIGS. 2 and 3. In particular, such analog optical systems may include a Mach Zehnder optical modulator and one or more photodetectors as shown in FIGS. 2 and 3. In particular, the system illustrated in FIG. 2 illustrates a basic Mach Zehnder based optical link while FIG. 3 illustrates a known analog optical link which includes a known complementary output Mach-Zehnder modulator and a balanced detector.
Such optical modulators are based upon the electro-optic affect. More particularly, certain materials, such as lithium niobate (LiNdO3), change properties, such as a refractive index, as a function of an applied electric field. This variance in the refractive index causes phase modulation of the light propagating through the material. This phase modulation of the optical carrier signal can be converted to optical intensity modulation by various optical devices including a Mach-Zehnder interferometer.
An exemplary Mach-Zehnder modulator is illustrated in FIG. 1. As shown in FIG. 1, the Mach-Zehnder modulator, generally identified with the reference numeral 20, includes an RF input port 22, an optical output port 24, two optical waveguides 26 and 28, an optical carrier input port 27 and a bias voltage port 29. In general, an optical carrier, such as a laser beam, is applied to the input port 27 and split into two paths defined by legs or waveguides 26 and 28. At least one of the legs 26, 28 is phase modulated by an RF input signal, applied to an RF electrode 30. The two legs 26, 28 are then recombined in an interferometer to provide an intensity modulated optical carrier at the output port 24. A bias voltage is also applied to the bias electrode 31 to control the bias intensity at the optical output port 24. The optical path length difference of the legs 26, 28 is a function of the physical path length and effective path length changes due to applied voltages (bias voltage plus small signal voltage) which change the relative index of refraction of the legs 26, 28.
Ideally an optical intensity modulator should have a linear relationship between its output optical power and the RF input voltage. As is known in the art, the output power curve of a Mach-Zehnder modulator is non-linear. Linearity and spur free dynamic range (SFDR) are important considerations in links using such modulators. The SFDR is the ratio of the largest to smallest signal power that the link can transmit and receive with a positive signal-to-noise ratio without introducing distortion (spurs) visible above the noise floor, as generally discussed in “Spur-Free Dynamic Range Measurements of a Fiber Optic Link with Traveling Wave Linearized Directional Coupler Modulators,” by Schaffner, et al. IEEE Photonics Letters, Vol. 6, No. 2, February 1994, pages 273–275, hereby incorporated by reference. Thus, many known linearization schemes for such systems are known to significantly degrade the link sensitivity to improve the SFDR. The sensitivity of simple lossy links can be improved by either lowering the switching voltage and/or using higher power optical amplifiers in the transmitter to get more optical power at the receiver's optical pre-amplifier input. For links requiring multiple optical amplifiers distributed along a long fiber optic channel or in a series of free space channels, the sensitivity can be improved by using higher power optical amplifiers or increasing the number of amplifiers and reducing their spacing in the channel(s). The sensitivity of links can also be improved by low biasing the modulator which results in a better relative small signal link gain with a fixed photo current.
Many known linearization schemes for Mach-Zehnder based links require at least two modulators requiring power splitting of the RF signal between the modulators. An example of such a system is disclosed in “Multi-Octave Operation of a Low Biased Modulator by Balanced Detection” by W. K. Burns, et al. IEEE Photonics Technology Letters, vol. 8, no. 1, January 1996, pages 130–132. Unfortunately, the degree of linearization in such configurations is highly dependent on the accuracy of the RF splitting ratios. Also, in such configurations, the SFDR is improved at the expense of sensitivity which increases the linearity requirements on the RF amp driving the Mach-Zehnder modulator.
Another approach to linearizing a Mach-Zehnder modulator is disclosed in U.S. Pat. No. 5,119,447. In particular, the '447 patent discloses a Mach-Zehnder modulator serially combined with a directional coupler. The Mach-Zehnder modulator is modulated by way of a pair of electrodes at a first polarity. The modulating signal is applied to a second pair of electrodes for the directional coupler and modulated at a second polarity opposite of the first polarity. Unfortunately, this system requires relatively precise RF power splitting between two or more electrodes in a combination Mach-Zehnder and directional coupler with a single laser. Thus, there is a need for a relatively linear Mach-Zehnder modulator which requires no RF power splitting. The sensitivity of this modulator is also degraded by over 6 dB.